To satisfy the demands on cars for a smaller size and lighter weight on one hand and a larger living space on the other hand, the reduction of space for the engine room has been required, which has led to the reduction in size and weight of the parts in the automotive electrical equipment and automotive auxiliaries, such as alternators, tension pulleys and the like. Also, in response to the demand for quietness, the engine room is closely sealed, so that greases are required to be resistant to high temperatures in consideration of the high-temperature operating environments.
In addition, the poly-V belts have been employed since the mid-1980s to meet the tendencies toward smaller-diameter pulleys and larger transmission torque, and to improve the belt durability. Concurrently, a peculiar problem has been produced, resulting from the flaking that occurs at the early stage, associated with white structural change on the rolling surface of the rolling bearings.
The bearings for use in the automotive electrical equipment or automotive auxiliaries have been thus required to have both long lubrication life and excellent resistance to flaking.
There are conventionally employed for rolling bearings lithium soap greases or diurea greases using inexpensive mineral oil as the base oil; lithium soap greases or diurea greases using as the base oil a synthetic hydrocarbon oil and an ether type synthetic oil and the like. In particular, diurea greases containing the aromatic urea compounds are frequently chosen in light of the durability under high temperatures.
However, those greases cannot satisfy the long bearing life under high temperatures because of the insufficient heat resistance of the employed base oils or thickeners and the poor flowability toward the bearing portions to be lubricated with grease.
In order to inhibit a catalytic action on the metal surface newly exposed as a result of the wear, an anti-flaking additive, for example, an oxidizer for passivation such as nitrites or the like is added to the grease composition for oxidizing the metal surface to inhibit the catalytic action thereof, thereby preventing the generation of hydrogen that would be caused by decomposition of the lubricant. (JP (Hei) 3-210394 A and JP (Hei) 5-263091).
Also, use of a phenyl ether type synthetic oil as the base oil for grease is proposed to prevent the generation of hydrogen caused by decomposition of the lubricant (JP (Hei) 3-250094 A).
Further, it is proposed to add an azo compound capable of absorbing hydrogen to the composition of grease to be used for the metal materials required to have tribological properties and for a variety of members, in particular, the grease to be enclosed in the bearing located at portions easily exposed to water (JP 2002-130301 A).
In addition, a grease composition comprising a fluorinated polymer oil as the base oil, polytetrafluoroethylene as the thickener, and an electroconductive material is proposed for the purpose of extending the life of rolling bearings, without causing the hydrogen embrittlement-induced flaking even when water permeates through the bearing (JP 2002-250351 A).
Also, there is proposed a grease composition comprising a poly α-olefin synthetic oil or diphenyl ether type synthetic oil, a urea-based thickener, at least one of an organic antimony compound or an organic molybdenum compound as the extreme-pressure agent, and zinc sulfonate (JP 2004-108403 A), which is designed to form a film on the surface of the rolling bearing to reduce the load applied to the rolling bearing in the tangential direction thereof under severe conditions including high temperatures, high speeds, heavy loads and the like.
However, any of the above-mentioned proposals do not provide sufficient measures against the flaking problem after generation of hydrogen because those proposals are not intended to cope with the action after generation of hydrogen, in other words, not intended to prevent hydrogen from penetrating to the inside of metal.
As the additives for grease, the compounds containing sulfur atom and nitrogen atom, such as thiadiazole compounds are known. For example, JP (Hei) 09-176670A discloses that a thiazole compound such as 2,5-dimercapto-1,3,4-thiadiazole and the like can impart a wear-resistant action when used in combination with an alkali metal borate.
JP 2002-206095 A discloses that when the grease to be enclosed in the rolling bearing for supporting the main shaft of machine tools comprises a predetermined sulfur-containing compound such as a disulfide compound or the like, the bearing life can be improved and heat generation can be reduced under the high-speed rotations.
JP 2007-186609 A discloses a grease composition for electric contact. When used within a low temperature region, the grease composition is capable of effectively reducing wear of the copper surface or copper alloy surface (silver-plated surface, gold-plated surface) without causing any chattering (drop in voltage) even at the contact of a very low current by adding an organic zinc compound or thiadiazole compound to the grease composition.